Electric resistance element and method of heat-treatment



-ered to .0000694 per temperature Patented Feb. 1,1949

ELECTRIC RESISTANCE ELEMENT AND METHOD HEAT-TREATMENT James M. Lohr,

Driver-Harris Company,

poration of New Jersey No Drawing. Application May 11, 1946,

- Serial No. 669,103 I 6 Claims. (Cl. 201-76) This invention relates to nickel-chromium alloys of the type employed as electrical resistance elements and more particularly to an alloy of this type having a low temperature coeflicient of resistance at temperatures below 300 C. and particularly in the range from 50 to 300 C.

Alloys consisting of 10 to 30 percent chromium and balance nickel have been extensively used in the manufacture of electrical resistance elements and have generally given satisfactory service. In various electrical devices requiring resistor elements of high accuracy, such as those used in modern radio, radar and the like, a resistance alloy having a temperature coefficient of resistance approaching zero, and a relatively high specific resistance is desirable and necessary. The normal temperature coefflcient of the standard 80-20 nickel-chromium alloy is plus .00014 ohm per degree centigrade. This is too high for "use in such apparatus.

Ithas been proposed to add aluminum and copper in amoimts of about 3 percent each to such alloys to lower the temperature coeflicient of resistance. 1 have found that such-additions lower the temperature coefllcient to. some extent but not to such a degree as is essentially desirable. I

If an alloy containing from 10 to 30 percent chromium, balance nickel, and more particularly the standard alloy of 80 percent nickel and 20 percent chromium, is annealed at a temperature approximately 1800- F., the temperature coefficient of resistance is lowered and its specific resistance is raised. Thus, when such an alloy is annealed at 1800 F. it has a temperature coeilicient of .0000842 per degree centigrade and a specific resistance of 612 ohms per circular mil foot. I have found that these properties can be further improved by a heat treatment from 1 to 5 hours at temperatures between 900 F. and 1100 F. Thus, if the said alloy is submitted to heat treatment of 1 hour at 900 F., the temperature coeflicient of resistance is lowdegree centigrade and the specific resistance is increased to 635 ohms per circular mil foot. By raising the temperature of the heat treatment to 950 F.. the temperature coeflicient of resistance is lowered to .0000678 and the specific resistance is increased to 639 ohms per circular mil foot. These experiments indicate that heat treatment of the standard alloy of 80 percent nickel and20 percent chromium at the temperatures mentioned lowers the ,coefilcient to some extent but again not to the degree desirable. The alloy, after being prepared, is drawn to produce wire of the desired size and then annealed by any of the known Morristown, N. J., assignor to Harrison, N. L, a corprocesses at a high temperature, preferably between 1700 and 1950 F. It is then heat treated for 1 to 5 hours at a temperature of 900 to 1100 F. and slow cooled, either in air or in the closed pot in which the heat treatment was made.

While some improvement may be obtained from such heat treatment of standard nickelchromium alloys, I have found that the temperature coefiicient of resistance may be still further decreased and the specific resistance may be still further increased by additions of relatively small amounts of aluminum. The aluminum maybe added in amounts varying from 2 to 4 percent. Excellent results may also be obtained by adding amounts of aluminum less than 2 percent or amounts greater than 4 percent and I do not confine my invention to these specific figures, but I have found that the best results are obtained within that range. Thus, in some instances the amount of aluminum added has been slightly more than 1 percent. The following results have been obtained from an alloy containing 20 percent chromium, 3.61 percent aluminum and balance nickel. Such alloy annealed at a temperature of 1950". F. had a temperature coeflicient of resistance of .000049 and aspecific resistance of 753 ohms per circular mil foot. Samples of this alloy were submitted to heat treatment for a period of one hour at temperatures from 800 F. to 1100 F. The sample submitted to a temperature of 800 F. had a temperature coefiicient of resistance of .000044 and a specific resistance of 737 ohms per circular mil foot. When heat treated at higher temperatures, the-temperature coeflicient of resistance was reduced to a minus figure. Thus, when heat treated at a temperature of 900 F. the temperature coefiicient of resistance was minus .00002'! and the specific resistance was 803 ohms per circular mil foot. After heat treatment at 1000'F. for 1 hour, the temperature coefiicient of resistance was minus .000036 and the specific resistance was 812 ohms per circular mil foot. When heat treated at a temperature of 1100 F., the temperature coefficient of resistance was minus .000022 and the specific resistance 846 ohms per circular mil foot. It is clear from these results that in this alloy by selecting the proper heat treating temperature the temperature coefficient of, resistance of the material can be reduced to zero over the range of temperature measured.

Similar results were obtained from an alloy containing 2.43 percent aluminum. This alloy,

when annealed, had a temperature coefiicient of resistance of .000068 and a specific resistance of I05 ohms per circular mil foot. After heat treating it for 1 hour at 900 F., the temperature aeeacao 3 coemcient of resistance was lowered to .000033 and the specific resistance was increased to 751 ohms per circular mil foot. After heat treatment of 1 hour at 1000 F.. its temperature coemcient of resistance was .000020 and its specific resistance was 761 ohms per circular mil foot. Heat treatment at a higher temperature slightly raised the temperature coefiicient of resistance to .000027 but also increased the specific resistance to 764 ohms per circular mil foot. In this alloy with a lower percentage of aluminum, the temperature coefficlent of the alloy is approaching zero as the heat treating temperature is raised. The lowest value appears at a heat treating temperature of 1000 F. and higher temperatures give less satisfactory results. Thus, an alloy containing between 2.43 and 3.61 percent aluminum when annealed and heat treated at temperatures from 800" F. to 1100 F, will have a temperature coeflicient of resistance closely approximating zero and by properly regulating the amount of aluminum additions and correlating the heat treatment, a zero temperature coefiicient of resistance could be obtained.

In some instances it is desirable to add iron or manganese and aluminum to the alloy. It may be found that a given addition of aluminum to a specific alloy will produce a minus temperature coeflicient of resistance and in many instances,

the temperature coeflicient of resistance can be brought nearer to zero more easily by additions of iron or manganese than by reducing the amount of aluminum. Thus, alloys having 'a comparable aluminum content to that of the alloy containing 2.43 percent aluminum maybe prepared containing relatively small amounts of iron which, while having a temperature coefiicient of resistance slightly higher than that discussed above, will produce alloys having a temperature coefilcient of resistance less than .00002. Such an alloy containing 2.32 percent aluminum and 1.5 percent iron has the following characteristics when annealed and heat treated: The alloy as annealed has a temperature coefiicient of resistance of .000077 and a specific resistance of 713 ohms per circular mil foot, When heat treated for 1 hour at a temperature of 900 F., the temperature coefiicient of resistance is lowered to .000059 and the specific resistance is increased to 747 ohms per circular mil foot. When heat treated to a temperature of 1000 F. for 1 hour, the temperature coefiicient of resistance is lowered to .000046 and the specific resistance is further 7 increased to 759 ohms per' circular mil foot. Heat treatment at a higher temperature, while resulting in an increase in specific resistance, also increases the temperature coefficient of resistance. Thus, with a heat treatment of 1 hour at 1100 F., the temperature coefiicient of resistance is .000054 and the specific resistance is 766 ohms per circular mil foot. While an increase in the iron content slightly increases the temperature coeflicient of resistance, the increase is not great.

Thus, an alloy containing 2.3 percent aluminum and 3.06 percent iron when annealed at 1950 F. has a temperature coeflicient of resistance of .000085 and a specific resistance of 716 ohms per circular mil foot. When-heat treated for 1 hour at a temperature of 900 F., the temperature coeflicient of resistance is .000061 and the specific resistance is 754 ohms per circular mil foot. Heat treatment for a period of 1 hour at a temperature of 1000 F. produced an alloy having a temperature coefiicient resistance of .000057 and a specific resistance of 764 ohms per circular mil foot V annealed at a temperature of 2000 and when the temperature was raised to 1100 F., the resulting alloy had a temperature coefiicient of resistance of .000066 and a specific resistance of 739 ohms per circular mil foot.

Examples of the advantages of the additions of iron to an alloy containing greater amounts of aluminum which would otherwise result in the production of an alloy containing a minus temperature coeificient of resistance are as follows: Asipointed out above, an alloy containing substantially 3.61 percent aluminum when heat treated had a minus temperature coeflicient of resistance varying from minus .000022 to minus .000036. By adding small amounts of iron to such an alloy, the temperature coefiicient of resistance can be raised to a small positive quantity. Thus, an alloy containing 3.37 percent aluminum and 3.10 percent iron annealed at a temperature of 1840 F. had a temperature coefiicient of resistance of .000068 and a specific resistance of 718 ohms per circular mil foot. When heat treated for one hour at a temperature of 900 F., the temperature coefiicient of resistance is reduced to .000036 and thespecific resistance increased to 763. ohms per circular mil foot, When heat treated for a period of one hour at 1000 F., the temperature coeflicient of resistance is reduced to .000012 and the specific resistance raised to 789 ohms per circular mil foot. When the tempera ture is increased to 1100 F., a temperature coeflicient of resistance of .000024 and a specific resistance of 792 ohms per circular mil foot is obtained. It thus appears that the best treatment for this alloy is at a temperature of 1000 F, for a period of one hour.

The iron content may be further increased without materially increasing the temperature coefiicient of resistance. Thus, with an alloy containing 3 percent aluminum and 6 percent iron, the following results were obtained: When annealed at 1950 F. and submitted to heat treatment of 900 F. for a period of 1 hour, the temperature coefiicient of resistance of the alloy was .000050 and the specific resistance was 786 ohms per circular mil foot. When the temperature is increased to 1000 F. for a period of 1 hour, the temperature coeflicient of resistance is .000022 and the specific resistance 839 ohms per circular mil foot. With a heat treatment of 1100 F. for a period of 1 hour, the temperature coefficient of resistance is .000035 and the specific resistance 838 ohms per circular mil foot. The best results therefore appear to be obtained when this alloy is heat treated for a period of 1 hour at a temperature of 1000 F.

Similarly the temperature coefiicient of resistance may be adjusted by additions of manganese to obtain a figure more closely approximating zero when the desired aluminum addition produces an alloy having a temperature coefiicient of resistance of a minus quantity. Thus comparing the values obtained as given above where approximately 3.5 percentaluminum is added to the basic nickel-chromium alloy with results obtained by manganese additions, the following figures are obtained. With an alloy containing 3 percent aluminum the addition of 1 percent manganese produced the following values: When F., the alloy hada temperature coefiicient of resistance of .000062 and a specific resistance of circular mil foot. By heat treating this alloy at a temperature of 900 F. for a period of 1 hour, the temperature coeflicient of resistance was decreased to .000019 and the specific resistance in- 703 ohms per creased to 766 ohms per circular mil foot. Heat treatment at a temperature 01.1000 F. for a period of 1 hour produced an alloy having a temperature coefficient of minus .0000024 and a specific resistance of v'7'1'1 ohms per circular mil foot. when the temperature of the heat treatment was raised to 1100 F., the temperature coeflicient of resistance was increased to .0000084 and the specific resistance was 1'78 ohms per circular mil foot.

The additions of manganese can be increased as high as percent without any material difference in the values obtained. Thus, an alloy containing 3 percent aluminum and 5 percent manganese annealed at a temperature of 1920 F.

had a temperature coetllcient of resistance of .00005'? and a specific resistance of 7'70 ohms per circular mil foot. When heat treated at a temperature of 900 F. for a period of 1 hour, the temperature coeflicient of resistance is reduced to .000013 and the specific resistance increased to 800 ohms per circularmil foot. At a temperature of 1000 F. for a period of 1 hour, the temperature coeflicient of resistance is .0000024 and the specificresistance is 816 ohms per circular mil foot. with a heat treatment of 1100 F. for a period of 1 hour, the temperature coefiicient of resistance is .000024 and the specific resistance is 809 ohms per circular mil foot.

In preparing the alloy when iron'is used as one of the addition agents, the nickel, chromium and iron are first melted and thoroughly deoxidized and degassified by the usual agents employed for this purpose. The aluminum is then added to produce the desired aluminum content without unnecessary loss. when manganese is used instead of iron, the nickel and chromium are first melted, deoxidized and degassifled. The manganese and aluminum are then added in this order. After the alloy is prepared, it is drawn to produce wire of the desired size and the wire is then annealed by any of the known processes at a temperature between 1700 and 1950 F. The wire is then heated for 1 to 5 hours at a temperature of 900 to 1100 F. and slow cooled, either in air or in the closed pot in which the heat treatment was made.

It will thus be seen from the foregoing discussion that by regulating the percentages of the addition elements, a zero temperature coeillcient of resistance, or one closely approximating thereto can be obtained and a specific resistance I treating the alloy for a in excess of 700 ohms per circular mil foot can I be obtained at the same time. The tests which have been conducted show that a heat treatment of the type set forth produces alloys having lower temperature coefficients of resistance and higher specific resistance than the alloy possesses before submission to the heat treatment. 7

While the nickel and chromium contents and the contents of the addition elements, iron or manganese and aluminum, have been given, it is of course understood that the alloy may contain small percentages of deoxidizing and degassifying agents including one or more of the elements of the group consisting of calcium, zirconium, boron, silicon, titanium, and where iron and aluminum are used as addition elements, small amounts of manganese. In the claims the term balance nickel is therefore intended to include such fractional percentages of deoxidizing and degassii'ying agents and also small tractional percentages of other elements usually found in such alloys. While an alloy containing 80 percent nickel and 20 percent chromium 'in the tests and an alloy referred to and was used containing 10 to 30 percent chromium and balance nickel has been more generally referred to, these percentages may be further varied without departing from the scope of the invention. The eflects of additions or aluminum, or or aluminum and iron. or or aluminum and manganese would be of the same order if wider variation from the -20 standard alloy were resorted to, but of diiierent specific values. When the alloy is stated to contain a certain percentage of chromium and balance nickel, the addition elements are added at the expense or the nickel content.

The term temperature coeihcient of resistance," as used herein, is intended to designate ohms per ohm per degree centigrade.

I claim:

1. An electrical resistance element comprising an alloy consisting essentially of 2 to 4 percent aluminum, .30 to 6.0 percent iron, 10 to 30 percent chromium, balance nickel, that has been annealed at a temperature of about 1700" F. to 2000 F. and heat treated for a period of 1 to 5 hours at a temperature of 900 F. to 1100' F., and having a temperature coefilcient of resistance below .00008.

2. The herein described method which comprises annealing an alloy containing 2 to 4 percent aluminum, .30 to 6.0 percent iron, 10 to 30 percent chromium, balanoenickel, at a temperature of about 1700 F. to 2000' F. and then heat periodofltofihoursata to 1100 F. to produce an coefllcient of resisthas been specifically temperature of 900 1''. alloy having a temperature ance below .00008. 7

3. An electric resistance element comprising an alloy consisting essentially of 2 to 4 percent aluminum, .30 to 6.0 percent iron, substantially 20 percent chromi balance nickel, that has been annealed at a temperature of about 1700 F. to

2000 F. and heat treated for a period of 1 to 5 hours at a temperature of 900 F. to 1100 F., and having a temperature coefiicient of resistance below .00008.

4. An electric resistance element comprising an alloy consisting essentially of substantially 3 percent aluminum substantially 3 percent iron, 10 to 30 percent chromium, balance nickel, that has been annealed at a temperature of about 1700" F. to 2000 F. and heat treated for a period of 1 to 5 hours at a temperature of 900 F. to 1100 F., and having a temperature coeilicient of resistance below .00008. v

5. An electric resistance element comprising an alloy consisting of substantially 2 percent aluminum, substantially 1.5 percent iron, 10 to 30 percent chromium, balance nickel, that has been annealed at a temperature of about 1700" F. to 2000" F. and heat treated for a period of 1 to 5 hours at a temperature of 900 F. to 1100 E, and having a temperature coeflicient of resistance below .00008.

6. The herein described method which comprises annealing an alloy containing substantially 3 percent aluminum, substantiallyB percent iron. substantially 20 percent chromium, balance nickel, at a temperature of about 1700 F. to 2000 F. and then heat treating the alloy for a period of 1 to 5 hours at a temperature of 900 F. to 1100 F. to produce an alloy having a temperature coeflicient of resistance below .00008.

JAMES M. LOHR.

(References on following page) REFERENCES CITED The following references are of record in the me of this patent:

' UNITED STATES PATENTS Number Name Date 859,608 Marsh July 9, 1907 2,048,166 Plllinget a1 July 21, 1936 2,071,645 McNeil Feb. 23, 1937 OTHER REFERENCES I Thum, Book ofstalnless Steels," 2nd edition. 1935, pages 466 and 503; pub. by Amer. Soc. for Metals, Cleveland Ohio. 

